1. Field of the Invention
The present invention relates generally to the fabrication of semiconductor devices and more particularly to a method of fabricating highly efficient, thin film semiconductor devices using a low quality substrate and low temperature, optical processing.
2. Description of the Prior Art
Thin film semiconductor devices, used extensively in a myriad of microelectronic and opto-electronic devices, have generated considerable academic and commercial interest in recent years. In particular, these "thin" film devices (i.e., solar cells), wherein the film thickness is less than the optical absorption depth, can exhibit high solar energy to electrical energy conversion efficiencies as compared to their "thick" film counterparts.
Optimal conversion efficiency for thin film silicon solar cells is generally thought to require a grain size of several hundred microns, preferably about ten times the film thickness, and a minority carrier diffusion length of about 50 to 100 .mu.m, or approximately twice the film thickness. In addition, to realize optimal efficiency, thin film solar cells must exhibit a variety of desirable optical and electronic properties. Such additional properties include low electrical resistivity, high optical reflectance, high optical confinement at the contact-semiconductor interface, and minimal absorption loss at the metal contact layer. Assuming the presence of all of these attributes, the theoretical efficiency limit for a single-junction, thin film multi-crystalline silicon solar cell is about 16% to 18%.
As interest in these thin film semiconductor devices intensifies, so too does the need for more efficient and economical designs. Unfortunately, simultaneous improvement in these areas, performance and cost, has been difficult due to a number of structural and functional limitations in semiconductor device fabrication. For example, most conventional semiconductor devices include costly, crystalline substrates which function as both the substrate for subsequent depositions as well as the semiconductor material itself. To make less expensive semiconductor devices requires the use of less costly, non-crystalline substrates. Unfortunately, most low-quality substrates (e.g., amorphous glass) cannot withstand the high processing temperatures required to produce a high efficiency film using conventional techniques. One conventional process for making silicon solar cells, for example, requires a thermal furnace with temperatures in excess of 850.degree. C. (the melting point of silicon is 1430.degree. C.). Such high processing temperatures place severe limitations on the choice of substrate (glass, for example, softens at about 500.degree. C.). In addition to their thermal lability, low-quality substrates typically include impurities which diffuse into the semiconductor material at high processing temperatures, thereby reducing the performance of the semiconductor or even rendering the device inoperative.
One way around the temperature limitation described above is to add a metal dopant, such as tin or copper, to the semiconductor material to depress the melting point, thereby facilitating deposition of the semiconductor at lower temperatures. This technique can work reasonably well for epitaxial growth on conventional crystalline substrates, such as single or multi-crystalline silicon. However, epitaxial growth of semiconductor materials directly on crystalline substrates (without a barrier layer) results in optical continuity (i.e., negligible index of refraction change) at the film-substrate interface. Unfortunately, optical continuity at the film-substrate interface reduces photovoltaic efficiency by enabling incident electromagnetic radiation to pass through the device, rather than being absorbed by the semiconductor material to produce electricity. Deposition of a metal-doped semiconductor on a non-crystalline or lattice mismatched substrate, on the other hand, can cause a number of problems including the formation of a very fine-grain semiconductor film, incorporation of residual metal at grain boundaries, and diffusion of metal at the interfaces. It is important that the semiconductor film have a relatively large grain structure to minimize the number of grain boundaries, or minority carrier recombination sites, which function as shunts to reduce flow of generated carriers between the semiconductor device and the external electronic circuit. Thus, although certain metals can effectively lower the melting point of the semiconductor material, which in turn reduces the deposition temperature and expands the choice of acceptable substrates, such techniques produce inefficient semiconductor devices, either because of optical continuity at the film-substrate interface or large carrier recombination losses at grain boundaries.
As previously mentioned, a grain size of several hundred microns is required for optimal conversion efficiency of thin film silicon solar cells. Unfortunately, conventional techniques can produce only small-grain silicon films at low processing temperatures. To effectively enlarge the grain size of this material, and thus reduce the number of grain boundaries or recombination sites, the film must be remelted and directionally solidified (i.e., re-solidified to position the grains in a columnar orientation) or annealed for very long durations. However, this grain-enhancement process requires extensive thermal heating which, as discussed above, prevents the use of most low-quality substrates.
Other electronic and optical properties affecting the performance of a photovoltaic semiconductor device include the device's optical confinement capacity, resistance due to electrical contacts, and optical reflectance. Forming thin, conductive metal layers on semiconductor materials is an essential step in the manufacture of microelectronic and opto-electronic devices to provide electrical contacts or current carrying paths to and from the semiconductor substance. During manufacture, such thin metal layers, or contacts, are applied to the semiconductor substance by any one of several well-known deposition techniques such as vapor deposition, sputtering, or electrolytic precipitation.
It is desirable to create an interface between the semiconductor material and the metal contact layer that has both low resistivity and high optical reflectance. Low resistivity is a primary requirement of any contact on a semiconductor device to reduce the barrier to carrier flow between the semiconductor device and the external electronic circuit. It is important, therefore, that the electrical contact be ohmic, even at very high current densities. Newer high efficiency solar cell designs utilize optical confinement techniques to capture and manage incident electromagnetic radiation so that more of it is absorbed in the semiconductor to produce electricity instead of escaping or being absorbed at the contact-substrate interface and dissipated as heat. Optical confinement is facilitated by highly reflecting interfaces where the contact joins the semiconductor material to prevent escape or absorption of the electromagnetic radiation at that interface and reflecting it instead back into the semiconductor for production of electricity. High optical reflectivity, therefore, is important to increase the amount of electromagnetic radiation energy absorbed by the semiconductor material, thereby improving the operation of the semiconductor device by increasing the number of photogenerated electrons available for collection. Unfortunately, it is difficult to provide both of these conditions simultaneously, low resistivity and high optical reflectance, at the semiconductor-metal interface using conventional methods. High optical reflectance requires a clean, abrupt semiconductor-metal interface, which is difficult to achieve. When a metal layer is deposited on a semiconductor, the contact is not electrically intimate, i.e., there is generally a layer of oxide which must be broken down by sintering or alloying. Conventional thermal processing produces a graded semiconductor-metal interface, which can provide a low-resistivity ohmic contact but does not provide high optical reflection, since the incident electromagnetic radiation at the interface can propagate into and through the graded interface to the metal and be totally absorbed and dissipated as heat.
While deposition techniques bond the metal layers to the semiconductor substance, the bond formed by such deposition techniques is usually not sufficient, either mechanically or electrically, to yield reliable, and sometimes even functional, devices. Consequently, the semiconductor must be processed further to improve the bonds between the conductive metal layer and the semiconductor material, for example, by either alloying or sintering the metal layers to the semiconductor surface, both of which involve additional high processing temperatures.
Sintering and alloying are two different processes that are used in semiconductor fabrication and which result in different electrical and mechanical characteristics, such as contact resistance, ohmicity, and bonding. Alloying generally creates a better bond and electrical contact with the semiconductor material because the metal layer and semiconductor material actually melt and meld together. Where semiconductor junctions or other thin film layers are close to the surface, however, such melting for alloying can disrupt or destroy the semiconductor structure or functions. Thus, a slightly lower temperature to produce sintering, which merely breaks down the interface oxide and remains more localized at the interface surfaces, may be more appropriate than alloying in those situations. Also, where there is heavy doping of the silicon, such as near a p.sup.+ -n or n.sup.+ -p junction, electrical contact between the doped silicon and the metal contact is easier to establish. Consequently, sintering, rather than alloying is usually sufficient. Therefore, it has become a general practice in the industry, particularly for solar cells and other opto-electronic devices, to alloy bottom metal contact layers to the bottoms of semiconductor substrates, while the top contacts, which are usually thin strips or grids near the junction, are only sintered to the heavily doped top surface of the semiconductor material.
In one conventional type of alloying process, metallized semiconductors, i.e., semiconductor substrates with metal contacts deposited thereon, are heated in an alloying furnace to a sufficiently high temperature to melt both the metal and the immediately adjacent substrate material, thereby improving the bond between the two materials and producing the desired electrical and mechanical characteristics. Such furnace alloying is frequently performed at temperatures in excess of 400.degree. C. and for as long as thirty minutes. Exposing the metallized semiconductor to such temperatures in a furnace over such an extended time heats up the entire semiconductor structure uniformly and creates an isothermal condition within the semiconductor. Unfortunately, such heating deep into the semiconductor material prevents the use of most low-quality substrates and, even with high-melting point materials, tends to degrade the semiconductor and reduce its performance.
Another problem with the furnace alloying process is that the atmosphere around the metallized semiconductor and the entire furnace are heated along with the individual metal layers or contacts and the semiconductor substrate. Not only does this heating of the atmosphere around the semiconductor waste energy, it also encourages degradation of the electrical characteristics of the metal layers due to atmospheric impurities. One form of this degradation is oxidation, which significantly degrades the electrical characteristics of the metal layer. It is well-known that even small traces of oxygen in the atmosphere surrounding the metallized semiconductor can oxidize the metal layer. Consequently, most furnace alloying processes require that the process be performed in a vacuum, or they require the use of a highly purified inert atmosphere, such as argon or helium, to reduce oxidation of the metal layer. Obviously, the creation of such special, purified environments is expensive.
Another, more popular alloying technique, commonly referred to as optical processing or rapid thermal alloying (RTA), bombards the metallic depositions on the semiconductor from the solid metal bottom side or from all sides, for a few seconds with pulsed, high intensity visible and infrared light, such as light produced by a high intensity CW visual light lamp. This pulsing of the metallized semiconductor with electromagnetic radiation results in a rapid increase in the temperature of the metal layer and the semiconductor substrate, thereby alloying the metal with the semiconductor substrate. U.S. Pat. No. 4,335,362 to Salathe et al., describes a slight variation of this RTA technique wherein narrow regions of the metallic layers are alloyed with the semiconductor by heating the regions with a focused beam from a Nd:YAG (four-level infrared) laser. Other examples of these techniques can be found in U.S. Pat. No. 4,359,486 to Patalong et al., U.S. Pat. No. 4,525,221 to Wu, and U.S. Pat. No. 4,566,177 to van de Ven et al.
Unfortunately, however, RTA suffers some of the same drawbacks as the furnace alloying process. For example, the RTA process usually illuminates a semiconductor device from both the top and bottom sides, the bottom side typically being the solid metal base or contact. Because the metal deposited on the bottom side is reflective, the outside surface of the bottom metal layer reflects a substantial portion of the incoming electromagnetic radiation back to the surrounding atmosphere, heating both the atmosphere and the outer surface of the metal. Of course, heating the outside surface of the metal enough to alloy the inside surface of the metal to the semiconductor material only exacerbates the contamination and oxidation problems described previously. Accordingly, the RTA process must also be performed under vacuum or in an inert environment. Even with such special inert environments, oxidation is still so prevalent in RTA that it is common to redeposit a metal layer over the alloyed metal in an attempt to re-gain the electrical integrity of the oxidized metal.
As discussed previously, "thin" film photovoltaic devices can exhibit high solar energy to electrical energy conversion efficiencies due to reduced minority carrier recombination in the bulk. It is important to minimize the minority carrier recombination sites in the semiconductor film, since recombination reduces the carrier flow between the semiconductor device and the external electronic circuit. In other words, fewer recombination sites provides an increased open-circuit voltage and fill factor, i.e., a higher quality and more efficient photovoltaic cell. Thus, because of their reduced volume, thin semiconductor films comprise relatively few recombination sites, and hence offer higher efficiencies, than thick films. Unfortunately, this reduced film thickness also means less absorption of optical photons. Thus, an important consideration in the fabrication of high efficiency thin films is to include some type of optical confinement technique to effectively increase the optical path length and hence absorption. Most existing techniques involve texturing surfaces of semiconductor devices, including masked chemical etching, unmasked chemical etching, and laser and mechanical grooving. Front surface etching has been used to reduce surface reflectance and to scatter incoming, normally incident light at oblique angles within the device. However, a disadvantage of front side texturing is that it causes an increased dark current, hence a lower open circuit voltage and fill factor. Also, texturing on the junction side of a semiconductor device, which is usually, but not necessarily always, on the front side, with previously used techniques can result in textured peaks that break the junction and cause shunting, which can seriously damage or destroy the device. Therefore, backside texturing is preferred over front side texturing, if other antireflective measures such as antireflective coatings are used on the front surface. Because light trapping is needed only for light that reaches the backside, i.e., less absorbing light, backside texturing can be quite effective.
Backside texturing, however, also has had its problems, one of the most significant of which is that the backside is usually used for the metal electrical contact. Conventional methods for producing backside texture without front texture involve chemical etching or otherwise texturing the back surface prior to deposition of the metal contact layer, which requires masking the front surface to avoid damage during the backside etching process. The device then has to be thoroughly cleaned before deposition of the metal contact layer. Also, the initial semiconductor-metal interface, while bonded in the deposition process, still has a high resistivity that must be lowered substantially, either by alloying or sintering, to be useful as an electric contact for the photovoltaic device. However, conventional alloying, even with the newer rapid thermal alloying (RTA) techniques, is difficult to control and tends to create a deep, graded alloy layer at the semiconductor-contact interface that absorbs light and dissipates the energy as heat, instead of reflecting the light back into the semiconductor material where it can be absorbed and converted to electricity.
Another problem commonly associated with the fabrication of thin film semiconductor devices is the high absorption loss at the metal contact layer as a result of the thin semiconductor film. Because the thickness of the semiconductor material in thin film devices is much less than the absorption length of incident electromagnetic radiation (i.e., complete absorption of long wavelength radiation requires a film thickness of several hundred microns), multiple passages through the semiconductor layer are required to effectively increase the optical path length and hence absorption of incident radiation by the semiconductor. Unfortunately, multiple passages require multiple reflections at the semiconductor-metal interface, which typically increases the optical absorption by the metal contact layer, resulting in the dissipation of energy as heat. Thus, although thin film devices can theoretically provide high photovoltaic conversion efficiencies because of the reduced minority carrier recombination in the bulk, devices based on conventional designs typically suffer from high absorption loss at the contact layer.
Given the number and variety of constraints in semiconductor device fabrication, it is not surprising that simultaneous improvement in device performance and cost is difficult to achieve. To make less expensive devices requires the use of low-quality semiconductor substrates which, unfortunately, cannot withstand the high processing temperatures used in conventional fabrication methods. Moreover, low-quality substrates typically include impurities which can diffuse into the semiconductor material at high processing temperatures, thereby reducing the performance of the semiconductor. Also, for optimal device performance, the design must provide light trapping and immunity to impurity diffusion.
Recognizing the need to reduce deposition temperatures, several researchers have used metal dopants to depress the melting point, thereby facilitating deposition of the semiconductor at a lower temperature, followed by thermal processing in an attempt to enhance the grain size. However, as discussed previously, this technique produces inefficient semiconductor devices, either due to optical continuity at the film-substrate interface or large carrier recombination losses at grain boundaries. Such methods are also expensive, with costly substrates and generally high production costs. Other researchers have investigated the use of ceramic materials as substrates, the composition and effectiveness of which are not known.
One researcher has attempted to reduce production costs by proposing a multi-junction semiconductor device using a glass substrate. Although this design features a thin semiconductor film and should provide high photovoltaic efficiency, it unfortunately suffers some of the same drawbacks as conventional fabrication methods. For example, formation of the junctions in this device may involve high processing temperatures, which can soften the substrate and allow diffusion of impurities from the substrate into the semiconductor material. Moreover, although this process incorporates a low-cost substrate, the overall production costs for this device remain high due to the expense associated with multi-layer depositions.
A need therefore exists for an improved fabrication process to produce high efficiency, low-cost semiconductor devices using low quality substrates and low processing temperatures. This high efficiency semiconductor should feature a variety of desirable optical, electronic and mechanical properties, including large grain size, low electrical resistivity, high optical reflectance, high optical confinement (or optical discontinuity) at the contact-semiconductor interface, minimal absorption loss at the metal contact layer, and an impurity barrier and gettering medium at the contact-semiconductor interface to prevent diffusion of impurities from the substrate into the semiconductor. Ideally, this high efficiency device should have a semiconductor film with a minority carrier diffusion length of about twice the film thickness, and a grain size of about ten times the film thickness. Until this invention, no such process or device existed.